Add back [ImplementedInBaseClass] IDL extended attribute

Source/bindings/scripts/interface_dependency_resolver.py

 # Copyright (C) 2013 Google Inc. All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 # notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 # copyright notice, this list of conditions and the following disclaimer
@@ skipping 125 matching lines @@
 
     No return: modifies dependency_interface in place.
     """
     merged_extended_attributes = dict(
         (key, value)
         for key, value in dependency_interface.extended_attributes.iteritems()
         if key in DEPENDENCY_EXTENDED_ATTRIBUTES)
 
     # C++ class name of the implementation, stored in [ImplementedBy], which
     # defaults to the basename of dependency IDL file.
-    # This can be overridden by [ImplementedAs] on the dependency interface.
+    # This can be overridden by [ImplementedAs] on the dependency interface,
+    # and omitted entirely by [ImplementedInBaseClass].
     # Note that [ImplementedAs] is used with different meanings on interfaces
     # and members:
     # for Blink class name and function name (or constant name), respectively.
     # Thus we do not want to copy this from the interface to the member, but
     # instead extract it and handle it separately.
-    merged_extended_attributes['ImplementedBy'] = (
-        dependency_interface.extended_attributes.get(
-            'ImplementedAs', dependency_interface_basename))
+    if 'ImplementedInBaseClass' not in dependency_interface.extended_attributes:
+        merged_extended_attributes['ImplementedBy'] = (
+            dependency_interface.extended_attributes.get(
+                'ImplementedAs', dependency_interface_basename))
 
     for attribute in dependency_interface.attributes:
         attribute.extended_attributes.update(merged_extended_attributes)
     for constant in dependency_interface.constants:
         constant.extended_attributes.update(merged_extended_attributes)
     for operation in dependency_interface.operations:
         operation.extended_attributes.update(merged_extended_attributes)